In general, in order to determine the status of deterioration of respective cells that constitute an industrial stationary battery system, the value of internal impedance or conductance is measured. An alternating current (AC) current injection method and a momentary load test; direct current (DC) current measurement are mainly used for measuring the values of the internal impedance and the conductance.
At this time, the value of the internal impedance can be expressed by Z=R+jX, the magnitude of the impedance value is calculated by extracting the square root of R2+X2, and the effective component of the impedance is expressed by the value corresponding the R component. Also, admittance that is the reciprocal of the internal impedance value is represented by Y=G+jB in which G indicates the conductance value.